G. CHEMICAL REACTIONS IN NATURE Similar colour changes may often be observed in nature. The acids of the exposed thallus cortex are not unfrequently split up by the gradual action of the ammonia in the atmosphere, one of the compounds thus set free being at the same time coloured by the alkali. Thus salazinic acid, a constituent of several of our native _Parmeliae_, is broken up into carbonic acid and salazininic acid, the latter taking a red colour. Fumarprotocetraric acid is acted on somewhat similarly, and the red colour may be seen in _Cetraria_ at the base of the thallus where contact with soil containing ammonia has affected the outer cortex of the plant. The same results are produced still more effectively when the lichen comes into contact with animal excrement. Gummy exudations from trees which are more or less ammoniacal may also act on the thallus and form red-coloured products on contact with the acids present. _Lecanora_ (_Aspicilia_) _cinerea_ is so easily affected by alkalies that a thin section left exposed may become red in time owing to the ammonia in the atmosphere. II. GENERAL NUTRITION A. ABSORPTION OF WATER Lichens are capable of enduring almost complete desiccation, but though they can exist with little injury through long periods of drought, water is essential to active metabolism. They possess no special organs for water conduction, but absorb moisture over their whole surface. Several interdependent factors must therefore be taken into account in considering the question of absorption: the type of thallus, whether gelatinous or non-gelatinous, crustaceous, foliose or fruticose, as also the nature of the substratum and the prevailing condition of the atmosphere. _a._ GELATINOUS LICHENS. The algal constituent of these lichens is some member of the Myxophyceae and is provided with thick gelatinous walls which have great power of imbibition and swell up enormously in damp surroundings, becoming reservoirs of water. Species of _Collema_, for instance, when thoroughly wet, weigh thirty-five times more than when dry[834]. There are no interstices in the thallus and frequently no cortex in these lichens, but the gelatinous substance itself forms on drying an outer skin that checks evaporation so that water is retained within the thallus for a longer period than in non-gelatinous forms. They probably always retain some amount of moisture, as they share with gelatinous algae the power of revival after long desiccation. Gelatinous lichens are entirely dependent on a surface supply of water: their hyphae—or rhizinae when present—rarely penetrate the substratum.